Lift-Off Process for Patterning of a Sputter-Deposited Thick Metal Stack for High Temperature Applications on 4H-SiC

With the rising need for power devices suitable for harsh environment conditions like high temperature applications, contact materials and packaging of the devices have become critical factors in device fabrication [1, 2]. Therefore, a contact metal stack containing silver and titanium nitride which can be used at elevated temperatures under oxygen atmosphere was investigated. For patterning of the approx. 2 µm thick sputter-deposited metal stack on the wafer front side, a lift-off process using a negative photoresist was established. Characterization of the photoresist sidewall shape was performed by cross-sectional views prepared with SEM and top view images taken on a microscope. It was found that for a successful lift-off, a distinct undercut is needed so no metal is deposited at the downside of the undercut, ensuring a metal-free surface for the solvent to reach the photoresist. To obtain this, most influencing factors are exposure dose and development time, which were optimized considering the undercut shape as well as pattern fidelity. Lift-off with acetone proved to be good for the fabricated 4H-SiC MOSFET devices.